1. Field of the Invention:
The present invention relates to a process for the preparation of 1,1,1-trifluoro-2,2-dichloroethane, namely, CF.sub.3 CHCl.sub.2, and, more especially, relates to the preparation of 1,1,1-trifluoro-2,2-dichloroethane by the photochemical chlorination of 1,1,1-trifluoro-2-chloroethane [CF.sub.3 CH.sub.2 Cl] under certain critically defined reaction parameters.
2. Description of the Prior Art:
1,1,1-TRIFLUORO-2,2-DICHLOROETHANE, IS A KNOWN COMPOUND. This chlorofluorocarbon is of value as an aerosol propellant, particularly as a stratospherically safe aerosol propellant. And the thermal chlorination of 1,1,1-trifluoro-2-chloroethane too has been described in the prior art. Compare E. T. McBee et al, Ind. & Engineering Chem., 39, 409 (1947), wherein the thermal chlorination of 1,1,1-trifluoroethane [CF.sub.3 CH.sub.3 ] has been shown to proceed in the following stepwise manner: EQU CF.sub.3 CH.sub.3 + Cl.sub.2 .fwdarw. CF.sub.3 CHCl + HCl (i) EQU CF.sub.3 CH.sub.2 Cl + Cl.sub.2 .fwdarw. CF.sub.3 CHCl.sub.2 + HCl (ii) EQU CF.sub.3 CHCl.sub.2 + Cl.sub.2 .fwdarw. CF.sub.3 CCl.sub.3 + HCl, (iii)
and wherein the reaction sequence [ii] can be said to represent the chlorination of interest.
However, McBee et at report that the molar ratio of chlorine to 1,1,1-trifluoroethane was approximately 1:1, and that the molar ratio of 1,1,1-trifluoro-2,2,2-trichloroethane [CF.sub.3 CCl.sub.3 ] to the subject 1,1,1-trifluoro-2,2-dichloroethane in their recovered product ranged from 1.1 to 1.4. This of course, reflects that the rate of chlorination of 1,1,1-trifluoro-2,2-dichloroethane to yield 1,1,1-trifluoro-2,2,2-trichloroethane is considerably faster than the rate of chlorination of 1,1,1-trifluoro-2-chloroethane to yield the desired 1,1,1-trifluoro-2,2-dichloroethane. Accordingly, there exists a need in the art to provide a facile process for the preparation of the valuable chlorofluorocarbon, 1,1,1-trifluoro-2,2-dichloroethane, with only minimal production of the more highly chlorinated by-product 1,1,1-trifluoro-2,2,2-trichloroethane.